Conventional steering of a wheeled motor vehicle is typically achieved by the vehicle operator (driver) rotating a steering wheel that is arranged in the passenger compartment of the vehicle to turn the steerable road wheels. Conventional steering systems generally include a rack and pinion type steering assembly operatively coupled to the road wheels and a steering column coupled between the steering wheel and the rack and pinion assembly for converting angular rotation of the steering wheel into a sliding motion on the rack to effect steering of the road wheels. In order to reduce the amount of driver effort (i.e., torque) that is required to rotate the steering wheel, conventional steering systems typically include a power assisted actuator that assists the operator with rotation of the steering wheel to overcome opposing forces such as road load forces on the road wheels and friction forces in the steering assembly. The amount of power assistance generally varies depending on the speed of the vehicle and the amount of effort applied by the vehicle operator to the steering wheel. Conventional power assist steering systems typically employ either hydraulic power assist or electric power assist. In contrast to hydraulic power assist systems, the electric power assist steering system offers variable assist capabilities, more efficient energy consumption, reduced mechanism complexity, increased reliability, and responsive on-demand steering assist, as well as other advantages.
The electric power assist steering (EPAS) system employs an electric motor for applying a controlled amount of torque to the steering assembly to assist the vehicle operator with angular rotation of the steering wheel. The conventional electric power assist steering system is generally configured with a feedback control system that electrically amplifies the driver steering torque input to the steering system to realize improved steering comfort and performance. The electric power assist steering system typically includes a rotatable steering wheel, a steering column, a rack and pinion assembly, a gear box assembly, and an electric motor. The conventional electric power assist steering system also employs a pinion torque sensor, as well as various other sensors. The pinion torque sensor is generally located between the steering column and the rack and pinion assembly and senses the amount of torque applied at the pinion. The measured pinion torque serves as an approximation of the input torque applied to the steering wheel by the vehicle operator and is commonly used to determine the amount of torque assist to be provided by the electric motor. The amount of torque assist is typically calculated from a tunable non-linear boost curve which generates a control command signal to control the electric motor to achieve the desired level of power steering assist.
Conventional electric power assist steering systems generally employ control systems that rely on intuition and trial and error tuning of either proportional-integral-differential (PID) controllers or lead-leg controllers. Due to inevitable modeling errors, sensor noises, and external disturbances, the steering system controller generally must perform robustly in the presence of such uncertainties. In order to match the performance of the electric power assist steering system, the non-linear boost curve output often requires a very large slope in order to achieve the optimal desired steering feel. To control this type of steering system at a very high gain with the boost curve, it is typically difficult for conventional steering controllers to maintain the stability as well as the robustness, i.e., component deterioration, mechanical non-linearities, and road disturbances, of the closed loop control system.
A more recent approach to electric power assist steering is disclosed in U.S. Pat. No. 6,250,419, which is assigned to the assignee of the present application, and is hereby incorporated by reference. The aforementioned patent teaches a steering system employing a torque estimator for determining an estimated torque signal and an H-infinity controller coupled in a feedback path for generating a feedback signal which is combined with a feedforward signal to generate a motor control signal as a function of the estimated torque. While the above-described H-infinity controller based steering system provides enhanced stability and robustness (e.g., component deterioration, mechanical non-linearities, and road disturbance), the steering system may not always achieve a desired steering feel that is comparable to the steering feel achieved with a conventional hydraulic steering system due to non-linearities in the system. This is at least in part due to the fact that non-linearities in the system and the center of gravity of the steering wheel may vary as a vehicle operator rotates the steering wheel while driving. Additionally, the steering column stiffness generally varies based on the steering angle and vehicle speed.
As shown in FIGS. 6A and 6B, the vehicle steering wheel 12, which is generally circular (e.g., ring-shaped), has a center position 100 which is commonly centered above the center of the steering column shaft 102, typically by about 3 to 5 mm. The center of the steering shaft 102 is the point about which the steering wheel 100 rotates. In addition, the steering wheel 100 has a center of gravity 104 at a position which typically is below the center of rotation of shaft 102 when the steering 100 is aligned in the straight-ahead on-center position as shown in FIG. 6A. When a driver turns the steering wheel clockwise to an off-center position as shown in FIG. 6B, a pendulum centering moment results. The pendulum centering moment causes the steering wheel 100 to want to return to the on-center position. Further, there exists a moment of inertia that must be overcome when starting and stopping a turn of the steering wheel solely as a result of the shape and mass arrangement of the vehicle steering wheel 100. Accordingly, the steering feel in prior known electric power steering systems will vary when the steering wheel is positioned in on-center positions as compared to off-centered positions.
Accordingly, it is desired to provide for an electric power assist steering system for a vehicle that provides torque assist to the vehicle operator to achieve optimal steering performance. In particular, it is desired to provide for an electric power assist steering control system for controlling the amount of electric power assist in a manner that offers stability, robustness of the system and desired steering feel, without sacrificing steering system performance. It is further desirable to achieve optimal steering assist performance while improving on-center and off-center steering feel.